You’ve likely seen stories recently about how Apple (NASDAQ:AAPL) is now larger than Exxon (NYSE:XOM) — at least measured by market capitalization, a simple calculation that multiplies share price times shares outstanding. As of this writing, Apple has a market cap of $440 billion, while Exxon is around $405 billion.

But what investors should care about isn’t a battle over size. The right question to ask is: “Which stock is a better buy?”

Well, both picks have a lot going for them — mammoth reach, dominance in their industry and huge brands. But let’s see how they compare in several key metrics to get beyond which one is biggest and decide which one is best for your portfolio.

Revenue: Exxon

No doubt, Apple is a sales machine. Revenue has exploded in the last five years, to about $108 billion in fiscal 2011. Few companies are in the $100 billion sales club. Automakers like General Motors (NYSE:GM) and financial stocks like Warren Buffett’s Berkshire Hathaway (NYSE:BRK.B) largely populate the list. But guess who’s at the top of that list? Yup — Exxon, with over $450 billion in sales for fiscal 2011. Apple may sell a lot, but Exxon sells a lot more.

Revenue Growth: Apple

Even though Exxon leads now, there’s no guarantee that it will always be ahead of Apple. Quarterly revenue increases happen like clockwork at Apple, most recently with its report that fiscal first-quarter sales soared 73% year-over-year. When you consider that sales are up more than fourfold in the last five years — from around $24 billion in fiscal 2007 — despite the Great Recession, it’s hard to ignore Apple’s momentum. Exxon has been on the rise, too, but the truth is its $450 billion in revenue last year wasn’t even its best ever. The total still lags the $477 billion Exxon tallied from record oil prices in 2008.

Profits: Exxon

Apple’s fiscal 2011 earnings were some of the best in the history of Corporate America – almost $26 billion in profits during just one fiscal year. That’s a staggering amount. However, it doesn’t even come close to Exxon. It has recorded five of the seven largest corporate profits in U.S. history — with its fiscal 2011 total of $41 billion almost doubling Apple.

Profit Growth: Apple

Again, though, we must return to the history of these companies to see which has more growth. For instance, despite the $41 billion in profits for the 2011 fiscal year, Exxon’s top tally was $45 billion in 2008. Sure, profits have been on the rise — but they’re rebounding more than growing. Apple, on the other hand, has seen profits go nowhere but up in recent years. Earnings are nearly four times higher in roughly the same period — from $13 billion in fiscal 2008 to $44 billion in fiscal 2011. Exxon may be regaining speed, but Apple never slowed down.